The present invention concerns a pervaporation device for the desalination of salt water comprising a channel or cavity for holding or transporting salt water and a liquid-water-impermeable, water-vapour-permeable membrane.
Pervaporation devices for the preparation of fresh water from salt water (i.e. desalination) are known in the art. International patent application WO 94/28706 discloses an irrigation mat comprising at least four layers (see especially FIGS. 1 and 2, in conjunction with page 5, line 7 to page 7, line 13): a top sheet made of a water-impermeable material, such as PE, PP, or PVC, a second sheet of a hydrophobic porous material, such as polytetrafluoroethylene, which does not allow the permeation of liquid water but does allow the permeation of water vapour and which forms with said top sheet a channel for containing salt water, and a third and a fourth sheet forming a channel for a cooling liquid which serves to condense the water vapour that has permeated through the second layer.
U.S. patent publication U.S. Pat. No. 4,698,135 discloses a desalinating drip irrigator xe2x80x9cpoweredxe2x80x9d by solar energy (see especially FIGS. 1 and 2 in conjunction with column 5, lines 11 to 65). The irrigator comprises an elongated evaporator plate (numeral 12 in FIG. 2) made of a hydrophobic porous material, which plate is supported above the soil by means of a pair of depending skirts (20). On top of the plate a plurality of elongated, parallel ducts (26) is provided for guiding salt water. The ducts are made of a solar radiation absorbent material, which is black for maximum retention of solar energy. Upon exposure to solar radiation, the salt water is heated, water evaporates and diffuses through the plate, and condenses on the soil below.
Unfortunately, during prolonged use of the said devices, it appeared that the hydrophobic porous films suffer from scaling and biofouling, which prohibits extended use or else necessitates costly maintenance.
International patent application WO 95/24260 concerns a device for, inter alia, purifying substances, e.g. waste water, by means of a membrane for separation by pervaporation. The liquid-water-impermeable, water-vapour-permeable membrane is non-porous and consists of a polymer based on hydrophilic copolyetherester (see the abstract on the front page and page 7, lines 13 to 27).
Finally, U.S. Pat. No. 5,595,662 discloses a desalination device comprising a cavity for transporting salt water. The device is made up of non-porous polymer materials, such as hydrophilic polyurethane or PEBAX. PEBAX is a polyether block polyamide in which the polyether and the polyamide segments are linked via ester bridges.
Although scaling and biofouling probably do not occur in the membranes according to WO 95/24260 and U.S. Pat. No. 5,595,662, these membranes were found to deteriorate fairly rapidly when exposed to heat and sea water and hence are unsuitable for use in desalination pervaporators.
It is an object of the present invention to provide a desalination pervaporator which does not exhibit the disadvantages of the prior art. This object is achieved by using a non-porous membrane which is made of a copolyether amide which comprises
(a) units of the following structure (I) 
and/or (II) 
wherein R1 has the meaning of an alkylene group with 3 to 11 carbon atoms which may be substituted or not, and R2 and R3 may be the same or different and represent a (cyclo)alkylene group with 4 to 11 carbon atoms which may be substituted or not or a difunctional aromatic group,
(b) polyoxyalkylene containing units of the following formula (III) 
wherein G stands for a polyoxyalkylene group and
(c) units of the following formula (IV) 
wherein R4 has the meaning of a (cyclo)alkylene group which may be substituted or not, a polyoxyalkylene group or a difunctional aromatic group, and wherein units (III) are linked to units (I) and/or (II) and/or (IV) via amide bridges.
It was found that the polymers according to the invention are exceptionally long-term resistant to the aggressive combination of heat and sea water, which makes them eminently suitable for use in solar heated pervaporation devices.
It is noted that DE 42 37 604 A1 describes the use of a membrane made of a polymer comprising polyether segments for ultrafiltration. Polyamide-polyether-block copolymers, such as PEBAX are mentioned as preferred polymers. Desalination, irrigation, and the problems relating to these fields of use are in no way suggested or disclosed. Furthermore, as stated above, membranes made of this polymer were found to deteriorate fairly rapidly when exposed to heat and sea water.
It is further noted that films made of the polymer used in the present invention are described in EP 0761715 A1. However, this reference is directed to a completely different field, viz. the use of this polymer in rainwear, tents, seat covers, underslating for roofing, waterproof shoes, mattress covers, medical purpose garments, and dressings. The problem of long-term stability of the polymer material in relation to sea water is not addressed in this reference and the use of a membrane made of this polymer in a pervaporation device is neither disclosed nor suggested therein.
In the following, the present invention will be described in detail.
As described above, the present invention relates to a pervaporation device for the desalination of salt water comprising a channel or cavity for holding or transporting salt water and a liquid-water-impermeable, water-vapour-permeable non-porous membrane wherein the membrane is made of a copolyether amide which comprises
(a) units of the following structure (I) 
and/or (II) 
wherein R1 has the meaning of an alkylene group with 3 to 11 carbon atoms which may be substituted or not, and R2 and R3 may be the same or different and represent a (cyclo)alkylene group with 4 to 11 carbon atoms which may be substituted or not or a difunctional aromatic group,
(b) polyoxyalkylene containing units of the following formula (III) 
wherein G stands for a polyoxyalkylene group and
(c) units of the following formula (IV) 
wherein R4 has the meaning of a (cyclo)alkylene group which may be substituted or not, a polyoxyalkylene group or a difunctional aromatic group, and wherein units (III) are linked to units (I) and/or (II) and/or (IV) via amide bridges.
As is known to the skilled person, an amide bridge is a structural unit of the formula 
Hence, e.g., unit (III) being linked to unit (I) via an amide bridge means that they form the following structure (the amide bridge is indicated in bold type): 
Preferably, the copolyether amide comprises 30-80 wt %, more preferably 40-70 wt %, and most preferably 55-65 wt % of units (I) and/or (II), and 20-70 wt %, more preferably 30-60 wt %, and most preferably 35-45 wt % of units (III), based on the total weight of units (I), (II) and (III) contained in the copolyether amide. It is further preferred that the polymer comprises units (III) and (IV) in a molar ratio of 0.8-1.2, more preferably 0.9-1.1, still more is preferably 0.95-1.05 and most preferably 1.0.
Unit (I) is generally formed by ring-opening of lactams. Suitable lactams are, e.g., gamma-butyrolactam, delta-valerolactam, epsilon-caprolactam, omega-lauryllactam or caprilactam, with epsilon-caprolactam being preferred. Generally, in the copolyether amide of the present invention, units (I) are linked to further units (I), thus forming blocks of their own, such as a polycaprolactam.
Unit (II) is generally formed by the reaction of a dicarboxylic acid and a diamine. Suitable diamines are tetramethylene diamine, hexamethylene diamine, diaminocyclohexane, 4,4xe2x80x2-diaminodicyclohexylene diamine, isophorone diamine, and 1,4-bisdiaminomethyl cyclohexane. Examples of suitable dicarboxylic acids are adipic acid, sebacic acid, terephthalic acid, isophthalic acid, 2,6-naphthalene dicarboxylic acid, and cyclohexane dicarboxylic acid. Generally, in the copolyether amide of the present invention, units (II) are linked to further units (II), thus forming blocks of their own. Preferred blocks are polyamide-6,6 and/or polyamide-6,12.
It is also possible to use combinations of units (I) and (II). Especially important are combinations of caprolactam and polyamide-6,6, with preference being given to compositions made up of caprolactam incorporating up to a maximum lo of 30 wt % of polyamide-6,6, or to compositions made up of polyamide-6,6 incorporating up to a maximum of about 40 wt % of caprolactam.
In unit (III), G has the meaning of a polyoxyalkylene group (also denoted as polyalkylenoxide). Generally, it has a molecular weight of 600-6000 and preferably of 1000-4000. Preferably the atomic ratio of carbon to oxygen in G ranges from 2.0 to 4.3.
Unit (III) can have, e.g., the following structure(s): 
wherein n is 25 to 40, preferably 30-40 and most preferably =xc2x133 (medium value) and/or 
wherein c stands for at least 1, (a+c) is at least 1, but less than 6, preferably xc2x15.5 (medium value) and b represents at least 10, but not more than 90, preferably 35-45 and most preferably xc2x137.5 (medium value).
One particular example of G is a polyethylene oxide. In this case, the copolyether amide preferably comprises 10-30 wt %, more preferably 15-25 wt % of ethylene oxide groups, based on total weight of units (I), (II), and (III) contained in the copolyether amide.
As stated above, R4 in unit (IV) has the meaning of a (cyclo)alkylene group which may be substituted or not, a polyoxyalkylene group or a difunctional aromatic group. Starting materials to be used for the formation of units (IV) are generally dicarboxylic acids. Such a dicarboxylic acid may be, e.g., an unsaturated dicarboxylic acid or a dicarboxylic acid having a functional group such as malic acid. The application of such a dicarboxylic acid makes it possible to incorporate functional groups into the polymer.
Also preferred are (cyclo)aliphatic dicarboxylic acids, wherein R4 has the meaning of a tetramethylene or cyclohexylene group. Other preferred units (IV) are those derived from a dimeric aliphatic fatty acid with at least 36 carbon atoms, such as with 44 carbon atoms.
If R4 stands for a polyoxyalkylene group, it may be, e.g., built up wholly or in part of ethylene oxide groups, and it has generally a molecular weight of 600-6000 and an atomic ratio of carbon to oxygen of 2.0-4.3.
For the preparation of the copolyether amides according to the present invention reference may be made to EP 0761 715 A1. Generally, the following procedure is applied: first, the monomer to be polymerised, preferably a unit (I) monomer and most preferably epsilon-caprolactam, is added beforehand to a nitrogen atmosphere containing a small quantity of epsilon-aminocaproic acid or monomers such as adipic acid and hexamethylene diamine, after which the dicarboxylic acid and a preferably equimolar quantity of the polyoxyalkylene diamine are added. Following flushing with nitrogen, the reaction mixture is then heated to a temperature in the range of 190 to 270xc2x0 C. over a period of 4 to 20 hours. Generally, the polymerisation reaction is carried out in the presence of a heat stabiliser such as 1,3,5-trimethyl-2,4,6-tris[3,5-di-tert.butyl4-hydroxybenzyl]-benzene and/or N,Nxe2x80x2-hexamethylene-bis(3,5-di-tert.butyl4-hydroxycinnamamide). In order to prevent the products being exposed to high temperatures for too long periods of time, which may result in potentially irreversible thermal decomposition, a catalyst may be employed if so desired. Frequently employed catalysts are sodium hypophosphite, phosphoric acid, and hypophosphorous acid. The polymerisation reaction usually is carried out until the resulting copolymer has a relative viscosity (measured on a solution of 1 g of polymer in 100 g of m-cresol of 25xc2x0 C.) of at least 2.0, preferably 2.4-2.8, after which the molecular weight is increased further by post-condensation in the solid state until a viscosity value in the range of 3 to 4 is reached.
A preferred embodiment of the present invention consists in an irrigation mat. Such mats can be used, e.g., in countries with insufficient rainfall and ample sunlight and sea water or other salt water sources, as a means to obtain fresh water. Such a mat comprises at least an upper sheet which is capable of absorbing sunlight and a lower sheet which comprises a liquid-water-impermeable, water-vapour-permeable non-porous membrane and forms at least one channel or cavity with the upper sheet, wherein the membrane is made of the copolyether amide as described above.
When this mat is filled with water and exposed to sunlight, the water in the mat will be heated to about 60 to 80xc2x0 C. and non-saline water vapour will emanate at the bottom. The water vapour is subsequently condensed into non-saline water. Thus, the mat will be able to desalinate and irrigate at the same time. At the end of the mat the remaining water, which has increased in salinity, will have to be drained.